A McDonald's restaurant in Rotterdam, the Netherlands, has a sophisticated anti-theft device. It sprays robbers with a fine mist containing artificial DNA that can be seen under ultraviolet light:

The new system involved an employee-activated device that sprays a fine, barely visible mist laced with synthetic DNA to cover anyone in its path, including criminals, and simultaneously alerts the police to a crime in progress.

The mist — visible only under ultraviolet light — carries DNA markers particular to the location, enabling the police to match the burglar with the place burgled. Now, a sign on the front door of the McDonald’s prominently warns potential thieves of the spray’s presence: “You Steal, You’re Marked.”

This is also on metafilter, where people hacked it in about five minutes. a story from my high school days: I got my edumaction at a greaser/banger school - we're talking FUBAR territory, big time. Library had a wee problem with book theft. So, school spent many many thousands of dollars on one of those electronic magnetic strip sensing devices. Keep in mind, this is in Saskatoon, well before teh infrawebby - these guys hacked it in about an hour or two. On their own. Started pranking the nerds, by pulling strips out, throwing them in their backpack so they'd be late for class. Some just elected to distract the librarian, and pass a book around outside the device to a friend.